Sonars are used to provide aural range information regarding target detection, range, and bearing to a human operator. They are used in both military and non-military applications. A diver-portable sonar that is commercially available is the aural output DHS-200 produced by Datasonics, Inc. since 1993, and produced by General Dynamics for the U.S. Navy prior to 1993. The three sonars used with the invention described herein were: a hand-built model built from spare parts, a General Dynamics sonar from the Federal Supply System, and a commercially available Datasonics DHS-200 sonar with one of its boards replaced with an older, identical board from a General Dynamics sonar because the original board was giving erroneous results. All further references to a sonar will be identified as the DHS-200 sonar, though in reality, all three sonars were used. The DHS-200 sonar produces a Continuous Transmission Frequency Modulated (CTFM) pulse that transmits a frequency sweep from approximately 145 kHz down to 115 kHz. Detection is accomplished aurally by the diver listening during each sweep for a difference frequency (or tone) between the transmitted pulse and a backscattered return from a target. The frequency of the tone determines the range a target is from the sonar. A low frequency tone indicates a target is close while a higher frequency tone corresponds to a target at a further range.
In an evaluation test of detection capability against bottom targets, detection performance was found to be highly dependent upon diver training and experience with a variable average probability of detection. However, a series of controlled measurements demonstrated that the DHS-200 sonar possessed the required sonar parameters (frequency, beam pattern, pulse type, etc.) needed to effectively detect and locate underwater targets. In these controlled measurements, the DHS-200 sonar's aural data were analyzed by viewing a video representation of the sonar's audio output with a spectral display. These measurements suggested that the poor results obtained in the evaluation test were not due to the acoustic performance of the DHS-200, but rather the method of relaying the acoustic information from the sonar to the diver. In other words, divers cannot solely rely on their hearing to detect and locate underwater targets. Consequently, the present invention modifies an existing hand-held sonar by complementing the current aural detection technique with a visual spectral display.